In prior art gallium arsenide MESFET devices, a critical parameter for the device structure is the product of the dopant concentration N times the square of the thickness of the active layer beneath the gate electrode (a.sup.2). Since the threshold voltage V.sub.T is proportional to Na.sup.2, prior workers have attempted to maintain good control over Na.sup.2 by etching the active layer to the desired thickness, such as is disclosed by Metze, et al. in "Gallium Arsenide Integrated Circuits by Selected Area Molecular Beam Epitaxy," Applied Physics Letters, Volume 37, No. 7, October 1980, pages 628-630. However, the tolerance on the resultant value of a is poor since this tolerance is both a function of the tolerance of the thickness of the deposited epitaxial layer and the depth of penetration of the etched region.
Another problem which arises in other prior art MESFET gallium arsenide structures is that the relative volatility of the arsenic component in the gallium arsenide semiconducting material alters the composition of that material at its external surfaces. For example, prior art MESFET structures wherein the region on each respective side of the gate is an exposed surface, the gallium arsenide surface possesses a high density of surface states near the mid band gap region. This phenomenon causes a surface state band bending condition to result so that the resultant gallium arsenide regions on opposite sides of the gate are in a partly depleted condition. This depleted condition causes a much higher resistivity to the regions surrounding the gate region than can be tolerated for good device characteristics. Attempts have been made to overcome this problem by ion implanting N+ highly conductive regions in the semi-insulating bulk surrounding the gate of the MESFET device. This approach to solving the problem, however, introduces processing difficulties inasmuch as a relatively high temperature of 800.degree. C. is required to activate the N+ dopant in the semi-insulating bulk.